Auto merge of #8632 - Jarcho:cast_ptr_alignment, r=llogiq

Don't lint `cast_ptr_alignment` when used for unaligned reads and writes

fixes #2881

Ideally this would trace the usage of the value rather than only looking at the parent expression, but that would require dataflow analysis. e.g.
```rust
let x = ptr as *const u16;
c.read_unaligned(x);
```

Arch specific intrinsic functions need to be checked for ones which could take an unaligned pointer. This can be another PR.

changelog: Don't lint `cast_ptr_alignment` when used for unaligned reads and writes

1 files changed, 66 insertions, 34 deletions

diff --git a/clippy_lints/src/casts/cast_ptr_alignment.rs b/clippy_lints/src/casts/cast_ptr_alignment.rs
index a4ef1344ab9..d476a1a7646 100644
--- a/clippy_lints/src/casts/cast_ptr_alignment.rs
+++ b/clippy_lints/src/casts/cast_ptr_alignment.rs
@@ -1,7 +1,6 @@
 use clippy_utils::diagnostics::span_lint;
-use clippy_utils::is_hir_ty_cfg_dependant;
 use clippy_utils::ty::is_c_void;
-use if_chain::if_chain;
+use clippy_utils::{get_parent_expr, is_hir_ty_cfg_dependant, match_any_def_paths, paths};
 use rustc_hir::{Expr, ExprKind, GenericArg};
 use rustc_lint::LateContext;
 use rustc_middle::ty::layout::LayoutOf;
@@ -20,45 +19,78 @@ pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>) {
         );
         lint_cast_ptr_alignment(cx, expr, cast_from, cast_to);
     } else if let ExprKind::MethodCall(method_path, [self_arg, ..], _) = &expr.kind {
-        if_chain! {
-            if method_path.ident.name == sym!(cast);
-            if let Some(generic_args) = method_path.args;
-            if let [GenericArg::Type(cast_to)] = generic_args.args;
+        if method_path.ident.name == sym!(cast)
+            && let Some(generic_args) = method_path.args
+            && let [GenericArg::Type(cast_to)] = generic_args.args
             // There probably is no obvious reason to do this, just to be consistent with `as` cases.
-            if !is_hir_ty_cfg_dependant(cx, cast_to);
-            then {
-                let (cast_from, cast_to) =
-                    (cx.typeck_results().expr_ty(self_arg), cx.typeck_results().expr_ty(expr));
-                lint_cast_ptr_alignment(cx, expr, cast_from, cast_to);
-            }
+            && !is_hir_ty_cfg_dependant(cx, cast_to)
+        {
+            let (cast_from, cast_to) =
+                (cx.typeck_results().expr_ty(self_arg), cx.typeck_results().expr_ty(expr));
+            lint_cast_ptr_alignment(cx, expr, cast_from, cast_to);
         }
     }
 }
 
 fn lint_cast_ptr_alignment<'tcx>(cx: &LateContext<'tcx>, expr: &Expr<'_>, cast_from: Ty<'tcx>, cast_to: Ty<'tcx>) {
-    if_chain! {
-        if let ty::RawPtr(from_ptr_ty) = &cast_from.kind();
-        if let ty::RawPtr(to_ptr_ty) = &cast_to.kind();
-        if let Ok(from_layout) = cx.layout_of(from_ptr_ty.ty);
-        if let Ok(to_layout) = cx.layout_of(to_ptr_ty.ty);
-        if from_layout.align.abi < to_layout.align.abi;
+    if let ty::RawPtr(from_ptr_ty) = &cast_from.kind()
+        && let ty::RawPtr(to_ptr_ty) = &cast_to.kind()
+        && let Ok(from_layout) = cx.layout_of(from_ptr_ty.ty)
+        && let Ok(to_layout) = cx.layout_of(to_ptr_ty.ty)
+        && from_layout.align.abi < to_layout.align.abi
         // with c_void, we inherently need to trust the user
-        if !is_c_void(cx, from_ptr_ty.ty);
+        && !is_c_void(cx, from_ptr_ty.ty)
         // when casting from a ZST, we don't know enough to properly lint
-        if !from_layout.is_zst();
-        then {
-            span_lint(
-                cx,
-                CAST_PTR_ALIGNMENT,
-                expr.span,
-                &format!(
-                    "casting from `{}` to a more-strictly-aligned pointer (`{}`) ({} < {} bytes)",
-                    cast_from,
-                    cast_to,
-                    from_layout.align.abi.bytes(),
-                    to_layout.align.abi.bytes(),
-                ),
-            );
-        }
+        && !from_layout.is_zst()
+        && !is_used_as_unaligned(cx, expr)
+    {
+        span_lint(
+            cx,
+            CAST_PTR_ALIGNMENT,
+            expr.span,
+            &format!(
+                "casting from `{}` to a more-strictly-aligned pointer (`{}`) ({} < {} bytes)",
+                cast_from,
+                cast_to,
+                from_layout.align.abi.bytes(),
+                to_layout.align.abi.bytes(),
+            ),
+        );
+    }
+}
+
+fn is_used_as_unaligned(cx: &LateContext<'_>, e: &Expr<'_>) -> bool {
+    let Some(parent) = get_parent_expr(cx, e) else {
+        return false;
+    };
+    match parent.kind {
+        ExprKind::MethodCall(name, [self_arg, ..], _) if self_arg.hir_id == e.hir_id => {
+            if matches!(name.ident.as_str(), "read_unaligned" | "write_unaligned")
+                && let Some(def_id) = cx.typeck_results().type_dependent_def_id(parent.hir_id)
+                && let Some(def_id) = cx.tcx.impl_of_method(def_id)
+                && cx.tcx.type_of(def_id).is_unsafe_ptr()
+            {
+                true
+            } else {
+                false
+            }
+        },
+        ExprKind::Call(func, [arg, ..]) if arg.hir_id == e.hir_id => {
+            static PATHS: &[&[&str]] = &[
+                paths::PTR_READ_UNALIGNED.as_slice(),
+                paths::PTR_WRITE_UNALIGNED.as_slice(),
+                paths::PTR_UNALIGNED_VOLATILE_LOAD.as_slice(),
+                paths::PTR_UNALIGNED_VOLATILE_STORE.as_slice(),
+            ];
+            if let ExprKind::Path(path) = &func.kind
+                && let Some(def_id) = cx.qpath_res(path, func.hir_id).opt_def_id()
+                && match_any_def_paths(cx, def_id, PATHS).is_some()
+            {
+                true
+            } else {
+                false
+            }
+        },
+        _ => false,
     }
 }